A color separation prism is constructed as shown in FIG. 3, in which an incident light S is separated into three primary color lights, i.e., green light Sg, red light Sr and blue light Sb, and the three primary color lights Sg, Sr and Sb are emitted from the color separation prism P in different directions. Since the color separating quality of a separation prism is substantial in the color reproducing quality of video cameras, etc, it is important to measure the exact color separating quality of a color separation prism. But no appropriate apparatus is yet commercially available for measuring the color separating quality and no standard measurement apparatus is established. So, manufacturers or researchers have made the measurement apparatus by themselves when necessary, which is a time consuming and troublesome work for manufacturers or researchers of video camera, etc.
A simple prior art method for measuring color transmissivities of a color separation prism is shown in FIG. 4. In this method, an integrating sphere I for integrating light before measuring the intensity of light by a photometer (not shown) is moved among three locations respectively on the paths of the three color light beams emitted from a color separation prism P. But this method requires careful and troublesome relocating operations of the integrating sphere I (with a photometer). When a two-beam method, in which a reference light beam is measured simultaneously with a measurement light beam transmitted through a sample, is used, the path of the reference light has to be moved according to the three locations of the integrating sphere I and the photometer. Thus the method of FIG. 4 is difficult to apply to the two-beam method.
Another prior art method for measuring color transmissivities of a color separation prism is shown in FIG. 5. In this method, six mirrors M1 through M6 are rectangularly arranged between a color separation prism P to be measured and an integration sphere I (with a photometer not shown). When the transmissivity for green light of the color separation prism P is measured, the two mirrors M1 and M2 on the central axis are removed and the green light separated by the color separation prism P and emitted straight therefrom enter the integrating sphere I without being reflected by any of the mirrors M1-M6. When red transmissivity is measured, the red light emitted down-leftward from the color separation prism P is reflected by the mirrors M3, M4 and M2 before entering the integrating sphere I. In case of blue transmissivity measurement, the blue light emitted upper-leftward from the color separation prism P is reflected by the mirrors M5, M6 and M2 before entering the integrating sphere I.
Since, in this method, the integrating sphere I is fixed, the two-beam method can be used unlike the method shown in FIG. 4. But the green light enters the integrating sphere I without being reflected by any mirror while the red light and the blue light enter the integrating sphere after being reflected by mirrors M2, M3, M4, M5 or M6. That is, the three primary color lights do not undergo the same optical history. Thus a 100%-transmission measurement has to be made for every color light, which is time-consuming. Another drawback of this method is that the exact absolute transmissivity can be measured only for the green light: only relative transmissivities as to the reflectivity of the mirror M1 can be measured for the red and blue lights and no exact absolute transmissivity can be obtained.